Patent GB-805795-A (GB)
Title: Improved apparatus for reshaping voltage waveforms
IPC: G05F1/42
Filed: 1956-04-09
Abstract: 805,795. Changing wave shape; automatic waveform control systems. CURTISS-WRIGHT CORPORATION. April 9, 1956 [April 14, 1955], No. 10772/56. Classes 38 (2) and 38 (4). Output alternating voltage E2 of a desired wave shape is produced from input E1 of another wave shape but of the same fundamental frequency by deriving a difference voltage from comparison of a reference voltage of the desired wave shape obtained by filtering E1 with a voltage having the shape of E1 or E2 and combining the difference voltage with E1 after any component of fundamental frequency has been removed from the difference voltage. For deriving a harmonic free wave E2, the reference voltage is a harmonic free wave derived by waveform generator 8<SP>1</SP> from the voltage across secondary winding 5<SP>1</SP> of transformer 3 energized by E1. The waveform generator may be a lowpass filter. The reference voltage and a voltage of the wave shape of E1 derived from transformer winding 6<SP>1</SP> are applied, appropriately phased, through resistances 11<SP>1</SP>, 12<SP>1</SP> to grid 9<SP>1</SP> of valve 10<SP>1</SP>, the output of which in winding 18<SP>1</SP> is their amplified difference. This difference which consists of the unwanted harmonics is injected into a winding 20<SP>1</SP> is such phase and magnitude as to cancel the harmonies in the supply. The effect of voltage of load frequency fed from winding 20<SP>1</SP> to the amplifier is cancelled by a voltage of that frequency induced in winding 16<SP>1</SP> and fed back to the amplifier through resistor 13<SP>1</SP>. To remove all traces of fundamental frequency from the difference voltage, a signal of difference voltage from winding 18<SP>1</SP> is supplied to windings 28, 35 of two-phase motors 27, 34 which respond to components of fundamental frequency respectively out-of-phase and in phase with E1 Motor 27 varies resistor 25 in phase displacement circuit 22, 23, 24, 25 to remove the out-of-phase component, and motor 35 varies resistor 31 to alter the amplitude of voltage supplied to 8<SP>1</SP> until the whole fundamental frequency is removed from the signal at transformer 17<SP>1</SP>. In a modification, Fig. 3, the reference wave is obtained by filtering the square wave derived from the output of a valve 40 whose grid is energized from E1 The amplitude of the square wave is maintained constant by the steadiness of the anode volts. Motor 27<SP>1</SP> varies resistor 46 in the supply to valve 40 to remove the out-of-phase component of the fundamental frequency, and motor 34<SP>1</SP> varies the voltage supplied from autotransformer 55 to boosting transformer 47 in the main supply. The motors may be omitted and' the fundamental removed from the reference voltage by combining it at the grid of a further valve 108 with the component, reversed in phase, left after the harmonics have been extracted from a sample of the reference volts at 107, Fig. 4 (not shown), Fig. 6. Fig. 6 shows a circuit in which the voltage for comparison with the reference voltage is derived from the output E2 at transformer 106, to give the stability of negative feedback. A further alternative to the motors may be a pair of phase-sensitive demodulator circuits, Fig. 5 (not shown).